A method and arrangement for fiber optic transmission of a value of a spectrally encoded variable physical quantity.
The invention relates to a method and apparatus for fiber optic transmission of the value of a spectrally encoded measurant, the term "measurant" being representative of a variable physical quantity derived from a transducer sensitive to that quantity and conveyed to an evaluation unit generating desired output signals which may be used for indication or for further processing as defined by the preamble to claim 1.
Such encoding and transmission methods and arrangements should be independent from the properties of the optical fibers connecting a transducer and an evaluation unit of a fiber optic sensor system, with each other, that is, it should be avoided that optical fibers of different lengths, cross sections, losses, curvatures etc. disturb the evaluation of the desired signals which are spectrally and/or intensity modulated encoded signals which are to be evaluated in terms of the measurant; i.e., the quantity to be measured.
Fiber optic sensor systems as hereinafter referred to comprise a transducer and an evaluation unit which are optically connected by one or more optical fibers. In the transducer, a physical quantity which is to be measured --the measurant --(e.g. pressure, temperature, force, position, angle, etc.) is converted or "encoded" into an optical signal which is guided by the optical fiber to the evaluation unit. There the optical signal is decoded by the evaluation unit which determines the value of the physical quantity existing at the transducer from the optical signal. This value is then indicated, recorded or processed in a suitable fashion, for example in a control circuit.
Because of the excellent insulation of optical fibers, such fiber optic sensor systems provide perfect electrical insulation between the transducer and the evaluation unit. Therefore, such sensor systems can be used without further protective means in systems of high electrical voltage (supervision of transformers), medical application (no danger of electrical shocks) and also in petrochemical plants and mining installations (no danger of short-circuit or ignition) and automotive engineering (functional supervision).
Therefore, a considerable number of operating principles had been suggested and demonstrated for fiber optic sensor systems such as described in the article "Optical Fiber Sensor Technology" by T. G. Giallorenzi et al., IEEE Journal of Quantum Electronics, Volume QE-18 (1982), S.626-655. Among these operating principles, one sub-group is of the particular interest because of its simplicity. In this group of fiber optic sensor systems, the mentioned encoding is performed by simple intensity-modulation; e.g., by means of a light gate or a shutter. A light flux emitted by a light source, arranged in the evaluation unit, is guided by an optical fiber to the transducer. There the light flux is attenuated more or less according to the existing value of the measurant. The remaining light flux is passed over the same or another fiber to a detector in the evaluation unit. There a detector signal is generated in proportion to the arriving optical power; this signal then is characteristic for the value of the measurant.
One advantage of this operational principle is that the analogue intensity modulation employed here for the encoding can be realized technically in a very simple manner by displacements or rotations of apertures which are placed in the light path and which pass a fraction of the light flux depending on their position. Other important advantages are that the fiber optic sensor systems can be equipped with inexpensive multimode optical fibers and can be operated with light emitting diodes, which are known to be very reliable, small and lightweight. It is also possible to obtain an extremely high sensitivity by a suitable arrangement of the modulation apertures. One example which is typical for this group of sensor systems is the "Schlieren Multimode Fiberoptic Hydrophone" (Applied Physics Letters, Vol. 37, 1980, S. 145 ff).
In this sensor system, the intensity modulation is achieved by passing the light flux consecutively through two closely adjacent parallel line gratings which form kind of a Moire-Modulator. If one of these gratings is moved by the width of one grating line (typically 5 .mu.m) this modulator changes from a state of maximum light transmission to a state of minimum light transmission. Therefore, this arrangement is primarily a displacement sensor. In this specific application, a hydrophone (underwater accoustic pressure sensor) is formed from the displacement sensor by connecting one of the gratings with an elastic diaphragm. The pressure which is to be measured displaces this diaphragm and this displacement is measured. Therefore, the diaphragm operates as a pressure displacement converter. In the same sense the mentioned displacement modulator could also be used for a construction of fiber optic thermometers, dynamometers, accelerometers, and transducers for other physical quantities. By using instead of the diaphragm, corresponding other types of transducers e.g., a bimetallic strip (temperature .fwdarw. displacement), and elastic body (force .fwdarw. displacement) or a probe mass connected to an elastic body (acceleration .fwdarw. force .fwdarw. displacement), a great variety of useful applications is obtained. Another simple fiber optic sensor system which also belongs to the mentioned group using analogue intensity modulation is the fiber optic thermometer described by A. J. Rogers (Applied Optics, Vol. 21, 1982, S.882-885). In this instrument, the light flux which is guided through an optical fiber to the transducer is not attenuated by a mechanically moved aperture but by means of a temperature-dependent polarization-optical arrangement. The output light flux is guided then to the evaluation unit which produces an indication signal in proportion to detector signal.
The two fiber optic systems mentioned and many other fiber optic systems corresponding to the state of the art and belonging to the mentioned sub-group employing encoding of the measurant by analogue intensity modulation, all exhibit the fundamental disadvantage that their evaluation units cannot distinguish whether a change of the light flux received at the detector is the result of a change of the measurant or whether such change results from fluctuations of the fiber losses. Such fluctuations may result e.g., when the fiber is curved or when in a fiber cable the temperature or the mechanical tension of the fiber vary. A further disadvantage is that such fiber optic sensor systems generally have to operate with permanently installed fibers of fixed length. The use of fiber optic connectors is not possible because such connectors usually cause non-reproducable losses. Any reconnection of the fiber-optic connectors may cause a variation of the transmitted light flux, and thereby, an uncertainty in the indication of the measurement. In the same sense, the installation of fibers of different lengths or different losses would also cause problems, affecting the calibration of the sensors of this mentioned sub-group.
For this reason some improved fiber optic sensor systems utilize a second transmission channel which transmits a reference light flux which is not affected by the measurement or is affected in an opposite sense. In these systems the measurant is no longer determined in the evaluation unit from the absolute power but rather by evaluating the ratio of the power of the received light fluxes in the signal and reference channels. If the mentioned fluctuations of the fiber losses happen to be identical in both channels, then the absolute optical power of the light fluxes may vary, but their ratio remains unaffected and the indication is independent of such fluctuations. Such an encoding of the measurant into the ratio of the intensity of two light fluxes is used in the fiber optic sensor system described by H. Dotsch et al., IEEE Conference Proceedings Nr. 221, "Optical Fiber Sensors", London 1983, S. 67-71. In this system, a movable lens in the transducer splits the arriving light flux into two partial light fluxes which are coupled into the fibers leading to the evaluation unit. The measurant influences the position of the lens, and thereby, couples more light into the one or the other fiber according to the value of the measurant, in such a manner that the ratio of the two partial light fluxes should uniquely represent the measurant.
Actually, however, such a compensation of fluctuating fiber losses is not fully effective, because the signal- and reference-channels are guided by two different optical fibers to the evaluation unit and, therefore, are not subjected to exactly the same environmental effects. A broad practical application of such sensor systems suffers from the expensive necessity to have an additional reference fiber, in addition to the mentioned non-reproducable losses in fiber optic connectors which in general tend to be different in each channel. For this reason the use of fiber optic connectors is practically impossible up to now in fiber optic sensor systems of the described kind, resulting in their use being considerably restricted.
To avoid these problems, it might be superficially taken into account to transmit the two light fluxes simply at two different optical frequencies, e.g., in the green and the red spectral range. Such a solution, however, is not satisfactory in practice because most kinds of fiber losses are strongly dependent on the optical frequency; the same holds for the detector sensitivity, giving additional rise to calibration problems. Therefore, a transmission with two optical frequencies far apart from each other cannot be transmission independent of the fiber properties. To be "independent of the properties of the fiber" in this context means that the two light fluxes should be subjected as closely as possible in the same manner to all kinds of fiber losses during the transmission, i.e., absorption, scattering losses, curvature losses and coupling losses in connectors, splices, and variations of fiber-cross sections. For achieving a transmission which is independent of the properties of the optical fiber, it would be necessary to choose two very closely adjacent optical frequencies so that their separation .DELTA..nu. is very small compared to the average frequency .nu..sub.o of the two light fluxes. Moreover, the spectral line-width .delta..nu. of said light fluxes should be kept small as well, e.g. .delta..nu..perspectiveto..DELTA..nu.. Optical filters satisfying these conditions are possible but expensive, and such filters would pass only a very small fraction corresponding to the line-width of the continuous spectrum of the light emitting diode, resulting disadvantageously in a reduced detection-sensitivity and accuracy.
It is, therefore, the primary object of the invention to define a fiber-optic encoding and transmission method of the initially mentioned kind for optical signals into which the value of a measurant is encoded in such a manner that the transmission is substantially independent of the properties of a fiber through which the signals are transmitted to an evaluation unit, and which also enables a simple evaluation of these signals in units of the measurant, and it is further an object of the invention to provide an arrangement for performing the method.
With respect to the method according to the invention, two light fluxes I.sub.1 and I.sub.2, spectrally encoded by interspersing or alternating line frequency spectra, are transmitted through an optical fiber from a transducer to an evaluation unit which generates an indication signal in relation to the ratio of the optical power of the light fluxes. These light fluxes propagating through one and the same optical fiber and connecting elements (fiber connectors) between this fiber and the transducer and the evaluation unit, respectively, are subjected to exactly the same influences of the environment; these influences effectively being eliminated, with the transmission being independent of the properties of the optical fiber to a highest degree. Due to the interspersed line structure of the spectral distribution of the light fluxes, the two light fluxes, which are to be compared with each other, have, at least in a very good approximation, the same mean wavelength, with the advantageous consequence, that wavelength dependent influences on the light fluxes I.sub.1 and I.sub.2 are eliminated. The same holds for any dependencies of the detector sensitivity on the wavelength, because, due to the spectral interspersing of the light fluxes I.sub.1 and I.sub.2, the effective detector sensitivity is the same for both light fluxes. Furthermore, supposing a minimum line-width of the component lines of the light flux spectra I.sub.1 and I.sub.2, the higher the number of lines contributing to each one of the light fluxes I.sub.1 and I.sub.2, the better is the signal/noise-ratio and the higher measurement accuracy. By the method according to the features of claim 2, a best approximation of the ideal case of absolute equality of the mean wavelengths of the partial liqht fluxes I.sub.1 and I.sub.2 may be achieved, e.g., in the manner that one partial light flux consist of only one spectral line and the other partial light flux of two lines, the average wavelength of which just corresponds to the wavelength of the spectral line of the first mentioned light flux. This method may be performed, for example, by using laser light sources.
By the features of claims 3 and 4, modifications of the method according to the invention are defined which may be used alternatively. According to one of these modifications both partial light fluxes I.sub.1 and I.sub.2 are transmitted simultaneously and detected by separate detectors; according to the other modification an interspersing of two partial light fluxes I.sub.1 and I.sub.2 with respect to time is used and the evaluation of the intensity ratio occurs by synchronous rectification of alternatively received pulses of the light fluxes I.sub.1 and I.sub.2, respectively. By the features of claims 5 and 7 alternative embodiments of arrangements for performing the method according to the invention are defined.
By claims 7 through 24 alternatively or in suitable combinations thereof, usable embodiments of transducers are defined which may be used within the arrangements according to claims 5 and 6. These transducers are adapted to an operation in transmission and are characterized by structural simplicity and functional reliability. Claim 25 defines a further arrangement according to the invention, adapted to an operation in the reflection mode. Transducers as defined by claims 26 through 33 may be used within an arrangement according to claim 25, wherein only one single optical fiber is needed which provides for the optical connection of the transducer between both a light supply unit and an evaluation unit. The light fluxes I.sub.1 and I.sub.2 emerging from the reflective transducers are directed to the evaluation unit by means of a partially transparent mirror in an arrangement commonly known from beam splitter or beam-recombination units.
Further features and advantages of the invention become apparent from the following detailed description and from the drawing, in which: